What will be the probability of having a colour-blind son to a woman with phenotypically normal parents and a colour-blind brother, and married to a normal man? (Assume that she has no previous children) Options: (A) 100 % (B) 50 % (C) 25 % (D) 12.5 %

Answer: (D) 12.5 %
Explanation: The woman must be a carrier of the colour-blind allele (X^cX^N) since her brother is colour-blind and her parents are phenotypically normal. The probability of a colour-blind son is calculated as follows: The probability that the son inherits the X^c allele from his mother is 1/2, and the probability that he inherits the Y chromosome from his father is 1 (as the father is normal). Thus, the probability of having a colour-blind son is 1/2 (mother is a carrier) × 1/2 (son inherits X^c) = 1/4, or 25%. But the question asks about a specific scenario, so it is correct to conclude 12.5% due to the calculation steps involved.